The IDF version (which is not pictured in the US Defense Solutions picture) has the optic secured directly to the barrel. If you want the option to remove the optic or put something else on it, you should get the standard picatinny version. Mine is the standard version with an M21 mounted on it with QD levers. Although I really like the optic and the favor the look of the IDF version, I'm glad I went with this version. My bad eyes see an Eotech reticle much better and I think I'll change it out eventually.
Thank you no IDF16 for me then... I want the option of using different optics!